Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
2000-06-30
2003-03-04
Le, Thanh Cong (Department: 2749)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C455S302000, C455S303000, C455S323000
Reexamination Certificate
active
06529719
ABSTRACT:
TECHNICAL FIELD
The present invention relates, in general, to the rejection of the image signal of a radio frequency signal converted to an intermediate frequency and, in particular, to the tuning or calibration of a frequency conversion mixer especially suited to be implemented on an integrated circuit.
BACKGROUND OF THE INVENTION
Unlike wireline communications, the wireless environment accommodates essentially an unlimited number of users sharing different parts of the frequency spectrum and very strong signals coexist next to very weak signals. A radio receiver must be able to select the signal of interest, while rejecting all others.
Among the important problems faced by the designers of radio receivers are image rejection and monolithic integration. A radio frequency receiver must be able to select the desired signal from its image. Otherwise, the subsequent detector circuit will be unable to distinguish between the desired signal and the image signal and, therefore, the output will be the result of the superposition of both. As wireless communications units evolve, means to reduce cost, size, and weight through monolithic integration are critical.
Image rejection relates to the ability of the radio frequency receiver to select the desired signal from the image of the desired signal spaced away by twice the intermediate frequency signal. This is important as the subsequent detector circuit will be unable to distinguish between the desired and image signals and, therefore, the output of the detector circuit will be a result of the superposition of both. This is the essence of the image signal problem.
In modern radio frequency receivers for wireless applications, when the problem of canceling unwanted image frequency response is handled in the mixer, typically 50 dB of image filtering is required from the overall system. This image filtering comes from a combination of pre-select band pass filtering, image filtering and possible use of an image reject filter. The high image rejection requirement means that the contribution of each circuit to the overall image rejection is critical.
FIG. 1
shows a conventional image reject mixer. An input radio frequency (RF
in
) is supplied to two mixers
10
and
12
. A first local oscillator signal (LO
1
) also is supplied to mixer
10
and a second local oscillator signal (LO
2
), in phase quadrature with the first local oscillator signal, is supplied to mixer
12
. Mixer
10
, responsive to the radio frequency input signal the first local oscillator signal, develops a first intermediate frequency signal and mixer
12
, responsive to the radio frequency input signal the second local oscillator signal, develops a second intermediate frequency signal. The intermediate frequency signals pass through low pass filters
14
and
16
and the first intermediate frequency signal is shifted in phase by 90° by a phase shifter
18
. The phase shifted first intermediate frequency signal and the second intermediate frequency signal are combined in an intermediate frequency combiner
20
which develops the output intermediate frequency output signal (IF
out
).
FIG. 2
shows a conventional intermediate frequency combiner that can be used in the
FIG. 1
image reject mixer. This intermediate frequency combiner has two differential amplifiers
22
and
24
. For the intermediate frequency combiner to function properly, differential amplifiers
22
and
24
should be in phase quadrature and should have equal gain. Differential amplifiers
22
and
24
are in phase quadrature when the pole frequency of feedback capacitor
26
in differential amplifier
22
is ten times lower than the intermediate frequency and the differential amplifiers have equal gain when the magnitude of the capacitance reactance in differential amplifier
22
is equal to the resistance of feedback resistor
28
in differential amplifier
24
.
Problems arise when there is degradation in image rejection in an image reject mixer due to process variations (i.e., variations in the values of components, such as feedback resistor
28
in differential amplifier
24
or feedback capacitor
26
in differential amplifier
24
in the intermediate frequency combiner).
SUMMARY OF THE INVENTION
To overcome the shortcomings of the prior ways of achieving image signal rejection considered above, a new image reject mixer is provided by the present invention. One object of the present invention is to provide a new and improved image reject mixer. Another object of the present invention is to provide a new and improved radio frequency receiver. A further object of the present invention is to provide a new and improved image reject mixer that is particularly suited for implementation in an integrated circuit.
Accordingly, an image reject mixer, constructed in accordance with the present invention, includes means for supplying a radio frequency input signal and means for supplying a first local oscillator signal and a second local oscillator signal in phase quadrature with the first local oscillator signal. Also included in this image reject mixer are a first mixer responsive to the radio frequency input signal and the first local oscillator signal for developing a first intermediate frequency signal and a second mixer responsive to the radio frequency input signal and the second local oscillator signal for developing a second intermediate frequency signal. An image reject mixer, constructed in accordance with the present invention, further includes means for phase shifting the first intermediate frequency signal by 90° and an intermediate frequency combiner for developing an intermediate frequency output signal. The intermediate frequency combiner has a first differential amplifier responsive to one of the second intermediate frequency signal and the 90° phase shifted first intermediate frequency signal and having a tunable reactance feedback path and a second differential amplifier responsive to the other of the second intermediate frequency signal and the 90° phase shifted first intermediate frequency signal and having a resistance feedback path. Also included in this image reject mixer are means for tuning the reactance feedback path of the first differential amplifier to place the first differential amplifier and the second differential amplifier in phase quadrature when the pole frequency of the reactance feedback path is at least ten times lower than the frequency of the intermediate frequency and to set the gain of the first differential amplifier equal to the gain of the second differential amplifier when the reactance of the reactance feed back path in the first differential amplifier is equal to the resistance of the resistance feedback path in the second differential amplifier.
It is to be understood that the foregoing general description of the invention and the following detailed description of the invention are exemplary, but are not restrictive of the invention.
REFERENCES:
patent: 5828955 (1998-10-01), Lipowski et al.
patent: 5901349 (1999-05-01), Guegnaud et al.
patent: 5903827 (1999-05-01), Kennan et al.
patent: 2002/0055347 (2002-05-01), Spargo et al.
Imbornone James F.
Mourant Jean-Marc
Abate Esq. Joseph P.
Cong Le Thanh
D'Agosta Stephen M.
RatnerPrestia
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